Many of the world's largest river deltas began building land around the same time, about 8,100 years ago, according to a new study published in the Journal of Coastal Research. The period when these deltas formed coincides with a time when the rate of sea level rise slowed.

If a slowed rate of sea level rise created the conditions to build coastal land, then a faster rate of sea level rise will intensify the delta's land loss, said Eugene Turner, a professor at Louisiana State University's College of the Coast and Environment and one of the study authors.

"It's like a checking account," he said. "If you don't have enough coming in and you have too much going out then you have a zero balance."

Turner and associates at the University of Maryland and Florida International University estimated the age of 36 of the world's largest river deltas based on an analysis of peat samples by the Smithsonian Institution. The decomposed plant samples were dug up from the river deltas in areas where land first formed. Radiocarbon dating was then used to determine the age of the samples.

Researchers found that the deltas began forming land around the same time as a gigantic glacial lake spanning much of Canada drained into the Atlantic Ocean. Lake Agassiz held more water than all of the Great Lakes combined, Turner said. The water drained from the lake when a natural damn made of ice broke. Fresh, cold water from the lake disrupted ocean currents and cooled the North Atlantic. Cooling of the ocean caused sea water to contract, decelerating sea level rise. The period of slowed sea level rise allowed the deltas to grow, Turner said.

Sea levels are rising today at about the same rate as when the deltas formed. But climate change is expected to increase the rate of sea level rise, putting the world's river deltas and the economies that they foster at risk, Turner said.

More than 500 million people on earth live on a river delta, according to a news release about the new study. The tipping point between delta resilience and collapse will likely occur within the next 50 years, according to the study. "It could be sooner than that," Turner said. "The history of making these predictions is that scientists tend to be conservative."